The present invention relates generally to integrated circuits and method of manufacturing and more specifically to a method of manufacturing self-aligned junction isolated complementary regions or wells in an integrated circuit.
The industry is constantly working toward increasing the device density on a wafer or chip. The number of devices that can be placed on a chip is limited by the size of the device and the electrical interaction. These place restraints on the spacing between devices. Further limitation are in the processing steps dealing with photolithography and the ability to form doped regions of the controllable size and impurity concentration.
In CMOS integrated circuit processes, where complementary transistors are the basic building blocks of complex circuits, doping concentration backgrounds of both N and P backgrounds need to exist for fabricating the transistors. Previous processes used only one photolithographic mask to implant an impurity to change impurity type in the silicon, or other type of substrate, for the well of one transistor while the background doping of the substrate would be the well of the complementary transistor.
In more advanced processes, devices are more sensitive to their well dopings and profiles so that two implant masks are needed to tailor the wells of each type of transistor. When a second masking step is used for twin wells, alignment tolerance must be included to prevent overlapping of implants. As illustrated in FIG. 1, the mask, which is the second mask, extends past the edges of the first P type impurity well and, thus, is greater than the opening in the first mask of FIG. 1. The size of the second mask is limited by the lithographic tolerances since overlap of the P- and N- regions is very undesirable.
The prior art has attempted to use silicon nitride mask and local oxidation to form a pair of masks using a single photolithographic process. In this process the first mask is silicon nitride and the second mask is the thick local oxidation which is an attempt at an inverse image of the silicon nitride. As is well known in the art, a bird's beak is formed, thus diminishing the accuracy of the mask alignments. Similarly, because of the high temperature cycle to form the local oxidation, undesirable redistribution of the first implant also results. Thus, this process has been found undesirable.
One technique to form a substantially reversed mask using one photolithographic step is to apply a thin metal layer over a thick photosensitive resist layer and selectively remove the photoresist layer with the thin metal layer thereon. Such processes involve a photoresist pattern with aspect ratio and treatment to the surface of the photoresist to give it an overhanging or undercut profile as a first mask. When metal as a second mask is deposited over this structure it will not be continuous over the step or a microcrack over the step will result. The discontinuity of metal or microcrack allows a wet chemical to attack the photoresist and lift off the unwanted metal. Because of the difference in height between the thickness of the metal layer and the photosensitive resist layer, a clean edge is not formed and, thus, some misalignment must be accounted for. Prior techniques generally use channel stops to account for any misalignment. This additional area occupies valuable space. A portion of the reverse masking step is illustrated in FIG. 2 after the implant usihg the first mask and the application of the thin second mask layer prior to the removal of the first mask layer and the superimposed second mask material.
Thus, it is an object of the present invention to provide a unique process for forming a truly reverse image mask.
Another object of the present invention is to provide a reverse image masking technique which allows the formation of truly self-aligned twin wells.
Still another object of the present invention is to provide a single photolithographic step, reverse masking technique which produces self-aligned twin wells.
These and other objects of the invention are attained by forming a first mask of a first material on a substrate and introducing first conductivity type impurities to form a first well region in the substrate. This is followed by applying a second mask layer of a second material to at least fill the openings in the first mask and cover the first mask. The second mask layer is removed sufficiently to reveal at least a portion of the first mask. The first mask with any superimposed mask layer material is selectively removed to form a second mask being the reverse image of the first mask. Second conductivity type impurities are introduced through openings in the second mask to form second wells aligned with the first well regions. The removal of the second mask material to reveal the first mask material may be achieved by removing the second mask layer to reveal at least the edges of the openings in the first mask layer. This reduces the amount of removal processing. Alternatively, a planarization layer may be provided over the second mask layer and the planarization layer and the second mask layer are removed down to a level to totally reveal the first mask layer which is then selectively removed.
The two mask layers must be of materials with sufficiently different characteristics such that they may be selectively removed. For example, the two mask layers may be selected from positive and negative photosensitive resist materials or a photoresist material and a metallic material. Where two types of photosensitive resist material are used, the first mask is formed using a positive photosensitive resist wherein the mask layer is the unexposed portion after developing. After the introduction of impurities, and before the application of the second mask material, the remaining first mask is radiation treated to form a hardened first mask layer. The second mask layer, after being applied, is removed to reveal at least portions of the first mask material which is then selectively removed with any superimposed portion of the second mask layer thereon. This forms the inverse mask. The second mask material may either be a positive or negative photoresist mask since the radiation treated and hardened portion of the first mask can be selectively removed even with the same photosensitive resist material used in the second layer. If it is negative photosensitive resist, radiation treatment may not be necessary.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.